A variety of supplemental recovery techniques have been employed in order to increase the recovery of viscous oil from subterranean viscous oil containing formations. These techniques include thermal recovery methods, waterflooding and miscible flooding.
Heterogeneous hydrocarbon containing subterranean formations, i.e., formations having relatively high permeability zones and relatively lesser permeability zones, are difficult to efficiently flood by secondary and/or tertiary oil recovery processes because fluids preferentially migrate into the highly permeable zones in the subterranean formations. Migration described above is undesirable when injecting treatment fluids into oil-containing formations for the recovery oil since the treatment fluids channel through the highly permeable zones bypassing the less permeable zones. The result is poor conformance and flow profiles of the treatment fluid in the formation. The hydrocarbons residing in the less permeable zones are not produced and the overall yield of hydrocarbons from the formation is reduced.
To increase the efficiency of formation flooding processes, the highly permeable zones in a subterranean formations are plugged or partially plugged to prevent or reduce migration of treatment fluids into them and to divert treatment fluids into adjacent, less permeable zones. In injection profile control projects, polymeric materials have been used in liquid slurries or suspensions to effectively enter and plug or partially plug the highly permeable and/or fractured zones of the formation. Fluids injected after such a treatment therefore move into upswept areas or zones of the reservoir which results in increased oil recovery.
Particularly attractive plugging agents include high molecular weight polymers such as polyacrylamides, polysaccharides and/or cellulosic polymers with multivalent metal ions and crosslinked products thereof. Many deep, hot reservoirs in the North Sea and Middle East are too hot for using conventional organic polymers, such as polyacrylamides and polysaccharides. Polysilicates and modified polyacrylamides may be used at moderate temperatures, but due to their fast gelling characteristics only limited injection is feasible. In addition, these polymers are affected by salinity and temperature extremes and are eventually rendered ineffective by chemical and thermal degradation.
U.S. Pat. No. 3,244,230 discloses a method for reducing the permeability of a deep permeable subterranean formation penetrated by a well by introducing into the formation through the well an aqueous solution of sodium silicate and urea. The aqueous solution of the sodium silicate and the urea react under the temperature conditions within the subterranean formation to form a hydrogel of silica that reduces the permeability of the formation.
U.S. Pat. No. 3,438,439 discloses a method of plugging formations having a sour gas cap containing hydrogen sulfide by forming elemental sulfur at the location in the formation where the plug or pancake is desired. An aqueous solution of sulfur dioxide is injected at the gas-oil interface which reacts with the hydrogen sulfide to form a layer of free sulfur.
U.S. Pat. No. 3,376,931 discloses a method for removing elemental sulfur from sour reservoir containing hydrogen sulfide gas and free sulfur using aqueous solutions of ammonium sulfide that dissolve the free sulfur. Removing the sulfur prevents sulfur precipitates from reducing the permeability of the formation, particularly adjacent the production well.
In my copending application Ser. No. 105,435, (docket no. 4503), filed Oct. 7, 1987, there is disclosed a method for selectively plugging the relatively highly permeable zones of a subterranean oil-containing formation having a neutral to alkaline pH by initially injecting an aqueous acid solution into the relatively high permeable zones that lowers the pH of the relatively highly permeable zones to about 6 or less. Next, an aqueous polysulfide solution is injected into the relatively highly permeable zones and allowed to undergo disproportionation under the pH conditions within the relatively highly permeable zones to form a precipitate of elemental sulfur in situ that plugs the relatively highly permeable zones in the formation. Thereafter, improved sweep efficiency is realized in displacing oil from the lesser permeable zones. If the pH of the relatively high permeable zones in the formation is about 6 or less, the initial treatment with the aqueous acid solution is unnecessary.
In the process of my invention I take advantage of the fact that the temperatures prevailing in the deep subterranean formation penetrated by a well are sufficient, in excess of 250.degree. F., to form a precipitate of elemental sulfur in situ from an injected aqueous solution of polysulfides which plugs the relatively highly permeable zones of the formation. Subsequently, injected displacing fluids are diverted into less permeable zones, facilitating recovery of hydrocarbons therein.